Terrain Analysis Package (TAP) Propagation
Comparison

Several
propagation models
are available in SoftWright's Terrain Analysis Package (TAP)™ software.These include the following:

Broadcast from FCC
Report No. R-6602, "Development of VHF and UHF Propagation Curves for TV
and FM Broadcasting," by Jack Damelin, et. al., September 7, 1966, and
Part 73 of the FCC Rules.

Careyfrom
FCC Report No. R-6406, "Technical Factors affecting the assignment of
facilities in the domestic public land mobile radio service,"by Roger B. Carey, June 24, 1964, and Part
22 of the FCC Rules.

Each
of these models represents an attempt to predict radio propagation as it is
affected by real-world conditions.A
useful comparison of several of these models is found in an article entitled
"Coverage Prediction for Mobile Radio Systems Operating in the 800/900 MHz
Frequency Range," by IEEE Vehicular Society Committee on Radio
Propagation, IEEE Transactions on
Vehicular Technology, Vol. VT-37, No. 1, February 1988.

The purpose of this paper is
to summarize some of the features of the models as implemented in the TAP
software.

Broadcast and Carey

The Broadcast (Part 73) and
Carey (Part 22) propagation models are based on the pertinent sections of the
U.S. FCC Rules and Regulations.Both of
these methods are essentially simplified statistical methods of estimating
field strength and coverage based only on a station's effective radiated power
(ERP) and height above average terrain (HAAT).Since the terrain information is averaged, neither model takes into
account specific individual localized obstructions or shadowing.Also, since the average used for these
models only includes the terrain between three (3) and sixteen (16) kilometers
from the transmitter site, terrain obstructions outside of this range are
ignored.This means that identical
results will be calculated whether or not a transmitting antenna has clear line
of sight or complete blockage by an obstruction in the first 3 kilometer
portion of a path.Likewise, any
terrain obstructions beyond 16 kilometers that block the line of sight to a
more distant receiving antenna are ignored.The main use for either of these models is for license applications or
other submissions to the FCC which specifically require the use of the methods
described in Part 73 or Part 22 of the FCC Rules, or other administrative
requirements, such as certain frequency coordination procedures..

On the other hand,
Bullington, Okumura, and Longley-Rice are more analytical models that consider
a number of other factors, such as individual obstructions (either terrain or
manmade), terrain roughness, etc.Okumura is often used in urban environments and includes correction
factors for various area types, such as urban, suburban, etc.Bullington considers individual obstructions
and computes losses, for example, for terrain obstructions, ridges, etc.Longley-Rice is a general model that
considers radio horizons and various environmental conditions.

Please note that this
discussion applies only to the SoftWright implementation of each of the models.Implementation of these models in software
other than TAP may be considerably different, at times even reducing field
strength calculations to over-simplified equations that do not afford the level
of accuracy provided in the particular model.This is often accomplished by setting certain parameters as fixed values
in the software.The SoftWright
implementation of the models gives the user a high degree of versatility and
the ability to independently select values for various parameters in each model
based on the particular circumstances and good engineering judgment.

Some of the original
engineering papers describing the models include various factors and
considerations that are not included in the TAP programs.For example, the SoftWright Bullington model
considers only knife-edge diffraction as discussed in the Bullington
paper.The Okumura program does not
currently implement the "Mixed Land-Sea Path" correction described in
the original Okumura paper.The details
of each specific SoftWright implementation can be found in the documentation
for each of the programs.

Note that the SMR (Specialized Mobile Radio) propagation model,
also available in TAP, is based on FCC Report No. R-6602 (as is the Broadcast model).
The SMR model "de-rates" the field strength values by 9dB to compensate for the
typically lower receiver antenna heights for mobile radios instead of the
30-foot AGL height in R-6602.

The SoftWright
implementation of the Bullington model uses terrain elevation information along
radials, and added obstruction file information if available for man-made obstructions,
vegetation, etc.) to compute knife-edge diffraction losses based on Figure 10
of the Bullington paper.The
obstruction penetration into the first Fresnel zone of the path is computed,
and the dB loss corresponding to the penetration is computed.Numerous losses can be computed for each
path, and the net received field predicted is equal to the free-space value
reduced by the sum of all the losses.Bullington is the model that is best used for air to ground
communications, since other models assume a receive antenna at a relatively low
height above ground (e.g., up to about 10 meters for Okumura).Bullington knife-edge diffraction loss
calculations have for years been the method used by the National Oceanic and
Atmospheric Administration for the protection of the Quiet Zone at Table
Mountain in Boulder, Colorado.New or
modified radio facilities in the area are required to coordinate with NOAA to
ensure that the RF noise floor at the Quiet Zone will not be raised.The recommended method of prediction is
Bullington, and measurements are taken after a facility is built to ensure
compliance.Years of documented field
strength measurements by NOAA at the Quiet Zone attest to the accuracy of the
Bullington model.

Bullington calculations
yield good results from about 80MHz and above, over rough paths when the
obstructions are primarily due to actual variations in the topography rather
that earth curvature alone.In other
words, on long paths that are relatively smooth, the path line of sight may be blocked
more by the "hump" of the curvature of the earth that by actual hills
or ridges that more closely approximate the model of a knife-edge
obstruction.In such cases, the loss
predictions by the Bullington model are generally not realistic.For example, for a 200 mile path over water
(completely smooth), Bullington may not be the best model to use.Several users have also reported unrealistic
results using Bullington at lower frequencies (40-70MHz), although the model
has a published minimum frequency of 30MHz.

The
Okumura model is based on a great number of measurements at various frequencies
in primarily urban areas in Japan.The
measured values and statistical methods were used to determine basic median
field strength, and numerous correction factors.The correction factors include adjustments for the degree of
urbanization, terrain slope and roughness, receiver location relative to nearby
hills and valleys, general street orientation in the service area, localized
obstructions, etc.While some of the
factors, such as terrain roughness, are computed automatically by the TAP
software based on the available terrain data, other parameters must be selected
by the user.The Okumura model is not
recommended for use below 150MHz, but is frequently used for cellular design,
800MHz systems, etc.

It
is important to select the type of area (urban, suburban, open, etc.) and other
factors carefully to accurately apply the model.The Okumura model is especially applicable in urban areas for
general coverage calculations where numerous discrete obstructions, such as
buildings, exist.It is important to
note that the definitions of "Urban", "Suburban", etc.,
described in the Okumura paper are in the context of Japanese cities,
especially Tokyo.The terms must be
applied with caution to areas in other countries to ensure similar building
density, type of construction, etc.The
IEEE article referenced at the beginning of this paper (comparing models in the
800/900 MHz range) suggests of the Okumura model, "The most typical
situation in the United States is far from the urban situation....Experience with comparable measurements in
the United States has shown that the 'typical' United States suburban situation
is often somewhere between Okumura's suburban and open areas.Okumura's suburban definition is more
representative of residential metropolitan areas with large groups of 'row'
houses" (p. 41).

The
Longley-Rice model is essentially a computer implementation of many of the
techniques described in Tech Note 101 (National
Bureau of Standards Technical Note 101, Transmission Loss Predictions for
Tropospheric Communication Circuits, by P. L. Rice, A. G. Longley, K. A.
Norton and A. P. Barsis, U. S. Department of Commerce, Revised January 1, 1967).Longley-Rice uses terrain information to
compute terrain roughness and radio horizons automatically.Other environmental variables must be
supplied by the user.These include
average climate conditions, soil conductivity, etc.While these factors can be set to custom values, the program
includes typical or average values that are applicable in most cases.Longley-Rice is a well established, general
purpose propagation model.TAP users
who expressed concern about Bullington results at lower frequencies (30-40 MHz
as discussed above) have reported significantly better results for field
strength calculations compared to field measurements when using Longley-Rice at
those frequencies.Longley-Rice is also
recommended where the path length and terrain smoothness (as discussed above
under Bullington) may be inappropriate for the Bullington knife-edge model.

The
Egli model is a greatly simplified model that assumes “gently rolling terrain
with average hill heights of approximately 50 feet” (Land Mobile Radio Systems, Edward N. Singer, PTR Prentice Hall,
1994, p. 196).Because of this
assumption, no terrain elevation data between the transmit and receive
facilities is needed.Instead, the
free-space propagation loss is adjusted for the height of the transmit and
receive antennas above ground.As with
many other propagation models, Egli is based on measured propagation paths and
then reduced to mathematical model.In
the case of Egli, the model consists of a single equation for the propagation
loss.

The
Hata/Davidson model is derived from the Hata model, which was in turn based on
the Okumura model (described above).

Hata (“Empirical formula for propagation loss in Land
Mobile radio services,” IEEE Transactions on Vehicular Technology, Vol. 29, No.
3, Aug 80) reduced many of Okumura’s adjustment to equations, but the equations
were limited to paths of less than 20km, as well as other limits on its
application.

Hata
and Hata/Davidson ignore some of the adjustment factors included in Okumura,
such as the slope of the terrain, street orientation, and correction for
location on hills.The main factors
included in Hata/Davidson are the area type (Urban, Suburban, Quasi-open, Open)
as well as corrections for the receiver antenna height.Hata/Davidson also includes frequency and
distance corrections to extend the limitations on Hata, particularly the
distance range to 300km.

It
should also be noted that Hata/Davidson computes the basic median field
strength (to which the various adjustments are applied) on the Height Above
Average Terrain (HAAT) of the transmit antenna (typically based on the 3-16km
segment of a path), rather than the topography of the entire path.

The
Rounded Obstacle transmission loss calculations are based essentially on two
sections of Tech Note 101 (“Transmission Loss Predictions for Tropospheric
Communication Circuits”, 1967, NTIS).Section 7 of Tech Note 101 involves the calculation of diffraction loss
over peak obstructions along the path.As to the determination of what constitutes an obstacle, TAP utilizes
its Obstacle Peaks method to programmatically assess which points along the
path represent critical obstructions in a manner similar to how a person may
intuitively “eyeball” peaks along a path.Section 9 of Tech Note 101 involves the computation of tropospheric
scatter losses due to these obstructions.Hence, the SoftWright Rounded Obstacle model identifies and “rounds” the
obstacles and then calculates associated diffraction and scatter losses for
your study.

Using
arrays of elevation values and a corresponding array of distances, the TAP
Obstacle Peaks method creates hypothetical “contours” or elevation bands
beginning at the highest elevation along the path.TAP then analyzes each point along the path and isolates a series
of points that enter and exit a band but do not penetrate the top of the band.The “peak” of this series of points is
assigned to the center of the range of points in the series and a radius of the
peak is calculated from the height and width of the series of points.TAP then determines the peak-to-peak path
horizons and applies a specific algorithm to discard intermediate peaks that
are deemed irrelevant based due to the geometry of the path.Diffraction loss is calculated for the
remaining path segments based on Section 7 of Tech Note 101 and scatter loss is
computed based on Section 9 of Tech Note 101.As described in more detail in the Rounded Obstacle article of your
Technical Reference Manual, TAP users have several options available related to
the use of diffraction loss, scatter loss or a combination of the two.

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